A series of cyber attacks against Japanese government Web sites this week caused no permanent damage but has prompted officials to re-examine their computer safeguards, the top government spokesman said Thursday.

Three attacks were launched on Tuesday and Wednesday, targeting public Web sites maintained by the prime minister's office and the Cabinet office, causing computers to freeze up under an overwhelming number of user requests that made it impossible for anyone to access the sites, Chief Cabinet Secretary Hiroyuki Hosoda told a news conference.

The so-called denial-of-service attacks bombard a Web server with so much data that the machine becomes unusable. "We had rather significant attacks," Hosoda said. "We must thoroughly check risk control measures not only at the Prime Minister's Office but other government offices as well." There was no significant damage from this week's attacks, because they were not designed to destroy programs, Hosoda said.

The government networks have returned to normal operations, he said. Officials are investigating the incident, but are having difficulty tracking the source of the attack. "We don't know whether the attack came from inside or outside the country," Hosoda said. In August and January last year, several ministries suffered similar attacks that temporarily froze their Web servers but caused no permanent damage.